The present invention relates to paper-manufacturing machines, and in particular to headbox systems for such machines.
The present invention relates particularly to structure for controlling the manner in which pulp stock flows in a hydraulic headbox.
The present invention is concerned particularly with a system for stabilizing the pulp stock flow in a hydraulic headbox which includes, sequentially in the direction of flow, starting at a distribution pipe system, a flow equalizing chamber, a system of turbulence passages, a tapering lip-cone passage portion extending from the latter passages toward the slice, and a flow-restriction location formed by a perforated plate situated at the junction between the equalizing chamber and the above system of turbulence passages.
Modern hydraulic headboxes utilized in paper machines, as known in the prior art, consist in general of a distribution header and distribution pipe system, a flow equalizing chamber, a system of turbulence passages and a lip-cone which is provided with an adjustable lip slice from which the pulp stock flow discharges onto the forming wire.
However, in the conventional equalizing chamber of such a headbox system, particularly if the latter chamber is of sufficient length, there is induced a state of flow which with time varies in its direction, in a relatively slow manner, before the flowing pulp stock reaches the system of turbulence passages. These directional variations form a considerable source of disturbance with respect to stability of the lip flow of the slice of the headbox. The physical phenomena which create these disturbances are referred to in greater detail below.
In connection with the prior art, in order to stabilize the headbox flow, it is known to guide the flowing pulp stock into the equalizing chamber by way of an array of several rows of tubes, so as to attempt to reduce in this way the variations in the direction of flow. Such a construction, however, has the drawback that the corrective effect achieved by way of this construction will be of a gradually diminishing magnitude as the equalizing chamber becomes gradually longer. In other words with such constructions the length of equalizing chamber, in the direction of flow, varies inversely with the corrective effect achieved by such a construction.
It is also known to provide equalizing chambers with Venturi-type throttling systems or with a set of plates which while extending in the machine direction nevertheless permit a transverse flow through gaps of these plates. Both of these types of systems do achieve some correction of directional variation in the flow, but the extent of correction is not sufficiently great to inhibit the change in the direction of flow to the desired extent.
It is furthermore known to place the interior of equalizing chamber in communication with an air-cushion type of damper. Since, however, the disturbance does not occur substantially as a pressure disturbance, the air-damping effect provides hardly any correction in the directional flow disturbance, regardless of whether or not the system of turbulence passages and the equalizing chamber have between themselves a certain angle. It has also been found that with an air-cushion damper acting at the intermediate chamber there may be within a certain flow range an undesirable increase in the instability of the flow at the lip or slice.
It is furthermore known to increase the extent of pressure drop in the system of turbulence passages by reducing the open cross-section thereof or by increasing the frictional surface area of the turbulence passage system. Reduction of the cross-section of flow at the input or inlet edge of the system of turbulence passages implies a reduction in the geometric aperture ratio and, as a result, a greater sensitivity with respect to variations of the constriction coefficient, in connection with which reference may be made to the formula presented below. There is experimental evidence which supports this latter effect. An appreciable enlargement of the area of the friction surface of the system of passages results in a construction which is prohibitively expensive.